Neal Horsley wants to kill abortion doctors. He publishes what is widely ­regarded as a hit list of medics and openly advocates the execution of women who have terminations.

But he is torn over this week's murder of one of the country's prominent abortion doctors. On the one hand, Horsley regards George Tiller's killing as "justifiable homicide" on behalf of the unborn and the man who shot him in the head at a church as a "soldier in a war".

"Tiller was a terrorist and I speak as a spokesman for those unborn who have been killed," said Horsley under posters of aborted foetuses at his office on the edge of Carrollton, Georgia.

"The thing about Tiller's assassination that was really appropriate is that they killed him in church. While he was there collecting the money, counting the money, his blood poured in to those thick carpets in that church. That was a fitting send off."

On other hand, vigilante killings are bad politics which Horsley says will not bring an end to abortions. So he has an alternative plan to rid the planet of Warren Hern. the doctor who is widely regarded as the next likely target on the anti-abortionists hit list.

Horsely is at the forefront of a movement to criminalise abortion and put men such as Hern on trial for mass murder, and then have the state execute them.

He is running for governor of Georgia and has drawn up what he calls the "Nuremberg list" of those deemed guilty. The names of doctors who have been murdered – five have been killed along with four abortion clinic workers, bodyguards and others in recent years – are struck through with a line. Those just wounded are listed in grey.

Horsley's strategy is one of the reasons that Hern is guarded by two armed US marshals at his fortified clinic in Boulder, Colorado. The windows are bulletproof after someone fired five shots through the front a few years ago. Access is through a reinforced door after photo identification is checked.

Hern, who is among the few doctors to do abortions far in to pregnancy – Tiller was another – is on Horsley's lists and another on a religious website that calls him "an enemy of life".

Those lists have nothing to do with calling for trials and everything to do with promoting murder says Hern. "Horsley wants me dead. They do not believe in the American legal system, they do not believe in the constitution, they do not believe in freedom. They don't want justice. They don't want a trial. They want death," he said.

"The main difference between the American anti-abortion movement and the Taliban is 8,000 miles and the Taliban is too civilised to kill people in a mosque. That Dr Tiller should be gunned down in his church while his wife was singing in the choir is a horrifying obscenity.

"We've lost much of our freedom and we work under a death threat. They come in the post,they call me, I've even had protests in front of the clinic where they prayed for me to die. No doctor helping other people should have to work under a constant death threat, armed guards, security protection. It's not possible to give that kind of protection to every single doctor."

Horsley has been at the forefront of an unrelenting campaign against abortion that straddles the blurry line between the mainstream movement characterising itself as "pro-life" and men like Scott Roeder who has been charged with Tiller's murder.

He describes his relationship with those who have used violence, and he's known a few, as akin to that between Sinn Fein and the IRA. It's an appropriate comparison because there are many, including Hern, who believe that although activists like Horsley are not pulling the trigger, they are bound up with the killers.

Horsley doesn't deny being closely associated with some of those who have murdered, including Paul Hill who was executed for shooting dead a Florida doctor and his bodyguard.

Horsley was a member of the extremist Party of God, through which he also got to know Eric Robert Rudolph who carried out a number of bombings that killed two people and injured scores of others as part of what he described as his "war to end this holocaust", referring to abortion.

Horsley regards them all as martyrs but says he advised them against vigilante killings as ineffective. Still he is angered that Roeder has been called a coward for shooting Tiller in the head. He says Roeder is a soldier fighting terrorism.

From computers and cameras in his small chaotic office – under a patchwork of posters including one of a tiny aborted foetus on the end of a finger and another of a large photograph manipulated to look as if it were taken from inside one of the twin towers as a plane headed toward it on 9/11 with the words: "Wake up call from the Lord" – Horsley puts out a barrage of propaganda.

There is a daily web cast and a 30-minute campaign advert shown on Georgia cable television in which he says: "That's why when people went out and assassinated abortionists I would be the first one to stand up and go: 'I'll tell you why they did it. I know exactly why they did it. I've been fighting against the impulse to go and do it myself for decades.'"

Horsley names Hern as one of those he would put on trial and see executed, along with Tiller if he had not been murdered.

"Let's prepare to do a Nuremberg trial for these people … this is not just a crime against little babies, this is a crime against all of humanity. It's a crime against the creator of all humanity," he said.

"I certainly believe that under the circumstances that Tiller went through, where there's no question about anything except the fact that he was making a lot of money over the killing of these babies, I would have sought the death penalty in his case. And probably would seek the death penalty in the case of any mother who decided to kill her child."

That is the kind of talk that has Hern and those who support the right to abortion accusing its more strident opponents of whipping up a frenzy of hatred that leads to murder. The Fox News host, Bill O'Reilly, who regularly spoke of "Tiller the killer" and accused him of running a "death mill", has had to fend off accusations that he bears some responsibility for the doctor's death.

Horsley is worried that he could be open to charges of incitement so at times he picks his words carefully. But then he suddenly gets carried away.

In the past he has said "We've always used lethal force if necessary to stop mass murderers", and that killing is sometimes necessary in the struggle to ban abortion.

Aren't many people going to interpret that as a justification of the murder of abortion doctors?

"I would think they were stupid if they interpreted it any other way. The fact is the best thing that Scott Roeder could get is for me to be on his jury. He'd walk out of that courtroom because if he killed that Tiller to stop him from killing those little babies, he walks. He's committed justifiable homicide in the same way that I would let him walk if he defended his own life," said Horsely.

The mainstream anti-abortion movement, principally represented by Operation Rescue, says it wants nothing to do with men such as Horsely and Roeder. Hern is having none of it.

He recounts the time that Operation Rescue's founder and then leader, Randall Terry, held a mass protest outside his clinic at which he led a prayer for Hern's death.

"The hideous hate speech of the anti-abortion fanatics has made it incredibly dangerous for doctors for 35 years. This is not new. This is not the act of single deranged gunman who simply decides to walk in to church and shoot somebody. This was a political assassination," he said.

"This is a national terrorist, totalitarian, fascist movement that is violent, well organised and well armed and they do not accept the basic premises of American society. Everything they've done for the last 35 years is designed to animate, to inspire, to give encouragement to anti-abortion assassins and anti-abortion violence.

"American people really have tuned out, they don't understand this. They want to believe this is some individual wing nut. No, No, No."

It's not just doctors who are targets. One of Horsley's most recent tactics is to film women who visit abortion clinics. He has mounted twin video cameras on the back of his pickup truck, which carries a hoarding declaring: "LEGALIZED ABORTION is Slavery to Satan. Stop It, or God Will Destroy the USA."

His supporters have sent hundreds of hours of film from clinics across America which is shown on his website. In some places live cameras broadcast from the front of clinics constantly. Horsley says it is an effective tactic because "shame is a deterrent to evil behaviour".

This wasn't always Horsley's view. Forty years ago he was a hippie and anti-Vietnam war protester who was jailed for two years for selling marijuana. But then he discovered religion and grew more passionately opposed to abortion. As his fanaticism grew, it cost him his marriage and caused an estrangement with his son.

Hern's marriage collapsed too under the pressure of the assaults and persistent harassments, including death threats on his home phone, abusive letters and periodic legal assaults.

Horsley says the vigilante killings are a mistake because they don't change much. He is preparing for a bigger and more bloody showdown, beginning with a run for governor in next year's election in the belief there is political mileage to be had in picking a fight with Barack Obama over abortion in a deeply conservative state such as Georgia.

"If they elected me governor I would, without hesitation, raise up a militia in Georgia. I would line them up along with me at the abortion clinic there in Atlanta and if the federal government came in and tried to escort people in to kill their babies, we would have a fight to the death," he said.

"There's some things so important that the only way they can be resolved is on the field of mortal combat where those on either side line up to kill each other till there's no more killing to be done. Look in my eyes. I'll fight to the death."